The present invention relates to the field of seismic data acquisition and processing. In particular, the invention relates to a system and method for seismic wavefield separation.
Optimal processing, analysis and interpretation of land seismic data ideally require full information about the wavefield so that the wavefield can be separated into its up- and down-going and P- and S-components as well as determining phase and polarity. For 3C acquisition of land surface seismic data it is common practice simply to interpret the vertical component as the P-section and the horizontal components as SV- and SH-sections. This xe2x80x9ctraditionalxe2x80x9d P/S interpretation is exact for vertical arrivals. However, as energy is incident away from normal incidence angles, this approximation breaks down, both because of projections on to all components, but also because of a non-unity reflection coefficients and mode-conversions at the free-surface.
Exact analytical filter expressions for wavefield separation have previously been derived by for instance Dankbaar, J. W. M., 1985, Separation of P- and S-waves: Geophys. Prosp., 33, 970-986, and these have been applied to seismic data in conventional recording geometries. Unfortunately, statics problems severely limit the practical use of these wave-equation based techniques.
Thus, it is an object of the present invention to provide a filtering technique that lends itself to be applied within local densely deployed single sensor receiver groups where statics are substantially constant.
It is a further object of the present invention to provide a filtering technique that can be implemented efficiently directly in the spatial domain.
According to the invention a method of creating a filter for use with locally dense seismic data is provided. The method includes obtaining survey geometry characteristics from a locally dense seismic survey designed to record characteristics of an elastic or acoustic wavefield. The seismic survey is made up of a number of groups of receivers, with each group comprising at least three receivers densely spaced from each other. According to the method, a filter is designed which uses spatial derivatives of the wavefield of order between 1 and the maximum order of spatial derivatives of the wavefield that can be estimated within a group. The filter is designed such that when combined with data from within a single group, the filter separates components of some or all of the wavefield arriving at the single group.
The filter can be designed so as to separate up/down going components, p/s components, or both up/down and p/s components.
The densely spaced receivers in the group are preferably spaced apart such that statics in the portion of the wavefield of interest are substantially constant. More preferably, each of the densely spaced receivers in the group are spaced apart by about 2 meters or less, or by a distance of about one fifth the shortest wavelength of interest or less.
Partial derivatives of the wavefield are also preferably calculated, and this can be done using a taylor series expansion as an approximation. According to the invention, the seismic data is preferably filtered by combining estimated near surface material properties, the seismic data, and the calculated partial derivatives (both in space and time).
The filter can also be used to separate surface waves or airwave induced ground motion from the seismic data. The free surface condition can be used to convert vertical derivatives of the wavefield to horizontal derivatives of the wavefield.
According to the invention, the seismic survey is performed primarily for the purpose hydrocarbon reservoir exploration, evaluation or characterisation, although other uses can be made.
The invention is applicable where the near surface velocities are isotropic or anisotropic.